1. Field of the Invention
This invention relates to an electrophoresis apparatus for separating and analyzing high molecular materials, such as proteins and nucleic acids which can have an electric charge in a solution, based on differences in their molecular charges and in their molecular weights.
2. Description of the Prior Art
The electrophoresis technique has been widely known for separating and analyzing proteins, nucleic acids, their degradation products and the like based on differences in molecular charges and in molecular weights in a sheet-like medium such as a gel membrane and a filter paper containing a buffer solution.
In particular, technique for determining base sequences of radioactive-labeled nucleic acids by using autoradiography is important in the field of genetic technology. In the electrophoresis for this purpose, a base-specific reaction product (mixture) of a radioactive-labeled DNA or of a fragment of such a DNA is subjected to electrophoresis along the direction of electric field of electrophoresis medium. In general, a plurality of base-specific reaction products are migrated in parallel along the direction of electric field. After this electrophoresis, a plurality of rows of electrophoresis patterns are obtained as an autoradiographic image (autoradiogram). Then, base sequence determination is carried out by comparing and collating these patterns therebetween.
Heretofore, an experimenter of electrophoresis has had to prepare by himself a gel membrane consisting of a hydrophilic high molecular material such as starch or polyacrylamide on a flat substrate such as a glass plate on every occasion for electrophoresis. This work has been a heavy burden to the experimenter who uses electrophoresis. Recently, in order to reduce this burden, a sheet device (gel sheet device) prefabricated for electrophoresis has become commercially available in which two rectangular, electrically-insulating, flexible sheets (a support sheet and a cover sheet) are disposed face to face sandwiching therebetween a spacer of a predetermined thickness at each edge in the width direction thereof, and a gel membrane is contained within the thus formed space as an electrophoresis medium.
Before the electrophoresis is to be started, it is necessary to pour the sample liquid, which is to be subjected to electrophoresis, into the upper edge of the gel membrane (i.e. the edge that is positioned at the upper portion of a vertical electrophoresis apparatus and does not face the aforesaid spacer). Therefore, forming of a plurality of rectangular wells or slots each provided with an open end at or in the vicinity of the upper edge of the gel membrane has widely been used in practice. However, this method results in a gap of 1 mm or more formed between a pair of adjacent electrophoresis zones (lanes), corresponding to the interval between the adjacent wells.
In the base sequence determination of a DNA, where electrophoretic images of DNA-fragments containing four kinds of bases (A, G, C, T) respectively, have to be compared and collated each other, such a gap between the lanes of electrophoretic images makes the collation difficult. In order to eliminate the gap between the electrophoresis lanes, instead of providing the wells (or slots), it has been known to dispose a shark's teeth comb, which is a flat plate-like member having protrusions like shark's teeth, such that the protrusions are in contact with or partially intruded into an edge of a gel membrane, and to pour a sample liquid into a substantially triangular space (pouring aperture) formed by the edge of the gel membrane and a pair of adjacent end faces of the protrusions. If the shark's teeth comb is not inserted accurately and steadily to the predetermined position, the sample liquid will invade a neighboring pouring aperture through the gap between the gel membrane and the protrusions of the shark's teeth comb. As a result, the electrophoresis patterns overlap each other, or the electrophoretic image is distorted, so that it becomes difficult to read base sequences.
In the base sequence determination of a DNA, it is desired that as much base sequences as possible be determined on a single electrophoresis medium. Accordingly, long-term electrophoresis for determining a sequence of high molecular components (for example, the number of discernible bases is within the range of 90 to 250 for four hours at 3,000 V) and short-term electrophoresis for determining a sequence of low molecular components (for example, the number of discernible bases is within the range of 20 to 110 for two hours at 3,000 V) are often carried out sequentially on a single electrophoresis medium.
In such a case, prior to pouring of a DNA sample liquid after the completion of the long-term electrophoresis, a new sample pouring site is usually washed with a buffer solution (of the same type as the buffer solution used in an upper buffer solution vessel of an electrophoresis apparatus). At the time of the washing, the shark's teeth comb is often deviated or separated from the predetermined position, so that the electrophoretic image is often distorted and base sequences become indiscernible. In addition, the edge face of the gel membrane swells toward the sample-pouring portion in the course of the long-term electrophoresis, so that the electrophoretic image obtained by the preceding short-term electrophoresis is disturbed.
Accordingly, it is important that, when the long-term electrophoresis and the short-term electrophoresis are carried out sequentially, the shark's teeth comb does not deviate or separate from the predetermined position, and the upper edge of the gel membrane is not caused to swell by the electrophoresis carried out previously.